Ten tennis players came to the competitions, 4 of them were from Russia. According to the rules for the first round, the tennis players are broken into pairs randomly. Find the probability that in the first round, all Russian tennis players will play only with other Russian tennis players.
In the triangle \(ABC\), the angle \(A\) is equal to \(40^{\circ}\). The triangle is randomly thrown onto a table. Find the probability that the vertex \(A\) lies east of the other two vertices.
At the power plant, rectangles that are 2 m long and 1 m wide are produced. The length of the objects is measured by the worker Howard, and the width, irrespective of Howard, is measured by the worker Rachel. The average error is zero for both, but Howard allows a standard measurement error (standard deviation of length) of 3 mm, and Rachel allows a standard error of 2 mm.
a) Find the mathematical expectation of the area of the resulting rectangle.
b) Find the standard deviation of the area of the resulting rectangle in centimetres squared.
At a factory known to us, we cut out metal disks with a diameter of 1 m. It is known that a disk with a diameter of exactly 1 m weighs exactly 100 kg. During manufacturing, a measurement error occurs, and therefore the standard deviation of the radius is 10 mm. Engineer Gavin believes that a stack of 100 disks on average will weigh 10,000 kg. By how much is the engineer Gavin wrong?
In a convex polygon, which has an odd number of vertices equal to \(2n + 1\), two independently of each other random diagonals are chosen. Find the probability that these diagonals intersect inside the polygon.
At a conference there were 18 scientists, of which exactly 10 know the eye-popping news. During the break (coffee break), all scientists are broken up into random pairs, and in each pair, anyone who knows the news, tells this news to another if he did not already know it.
a) Find the probability that after the coffee break, the number of scientists who know the news will be 13.
b) Find the probability that after the coffee break the number of scientists who know the news will be 14.
c) Denote by the letter \(X\) the number of scientists who know the eye-popping news after the coffee break. Find the mathematical expectation of \(X\).
A high rectangle of width 2 is open from above, and the L-shaped domino falls inside it in a random way (see the figure).
a) \(k\) \(L\)-shaped dominoes have fallen. Find the mathematical expectation of the height of the resulting polygon.
b) \(7\) \(G\)-shaped dominoes fell inside the rectangle. Find the probability that the resulting figure will have a height of 12.
Every Friday ten gentlemen come to the club, and each one gives the doorman his hat. Each hat is just right for its owner, but there are no two identical hats. The gentlemen leave one by one in a random order.
Seeing off the next gentleman, the club’s doorman tries to put the first hat that he grabs on the gentleman’s head. If it fits (not necessarily perfectly), the gentleman leaves with that hat. If it is too small, the doorman tries the next random hat from the remaining ones. If all of the remaining hats turned out to be too small, the doorman says to the poor fellow: “Sir, you do not have a hat today,” and the gentleman goes home with his head uncovered. Find the probability that next Friday the doorman will not have a single hat.
A table of size \(3 \times 3\) (as for playing tic-tac-toe) is given. Four chips are put (one each) on four randomly selected cells. Find the probability that among these four chips there are three that stand in a row vertically, horizontally or diagonally.
One day in autumn the Scattered Scientist glanced at his ancient wall clock and saw that three flies fell asleep on the dial. The first one slept exactly at the 12 o’clock mark on the clock, and the other two just as neatly settled on the marks of 2 hours and 5 hours. The scientist made measurements and determined that the hour hand does not threaten the flies, but the minute one will sweep them all in turn. Find the probability that exactly 40 minutes after the Scientist noticed the flies, exactly two flies out of three were swept away by the minute hand.